Low rank carbonaceous materials, such as brown coal, are a basic energy source in many regions of the world. The use of such materials for the provision of energy is often convenient due to their abundant supply and location in the vicinity of energy requirements. However, as a result of high moisture content of such materials they are of relatively low energy value and have relatively high mass to energy content ratio. This means that it is often not economically viable to transport such materials far from where they are deposited and that unless deposits are located close to the surface they may not constitute a viable energy source. Furthermore, the high moisture content of such materials means that their combustion is not clean and gives rise to more unwanted combustion by-products than combustion of higher rank carbonaceous materials. There is therefore a need to develop efficient means for reducing the moisture content of such materials.
For example, Australian Patent Nos. 561,586 and 588,565 disclose processes for treating brown coal by subjecting the material, with or without the addition of water, to shearing forces to produce a plastic mass, from which pellets are formed by extrusion. The pellets are then subjected to a drying step using air at or near ambient temperature.
Earlier work by the present inventor, as disclosed in Australian Patent No. 636,847 has involved the use of brown coal containing pellets as feed for the production of char by pyrolysis or in the reduction of iron ore. Such pellets when first formed need to undergo air drying to remove a significant proportion of water before undergoing high temperature treatment. The drying must be performed slowly so as to not undermine the structural integrity of the pellets, and can take up to several days. Moreover, because the pellets have a relatively low intrinsic economic value, capital, handling and operating costs should be minimised. Furthermore, the large tonnages of the pellets required to be dried, are typically stored in open air heaps and adequate air circulation to all pellets during drying can be problematic.
Different approaches to the drying of granular or particulate materials have been disclosed in U.S. Pat. No. 5,233,766 and International Patent Publication Nos. WO97/25295 and WO2004/048868. U.S. Pat. No. 5,233,766 discloses a dryer having a substantially vertical chute with a top and a bottom and at least two walls with air-pervious containment means for containing granular material, allowing an air stream to pass through at least portions of the chute. WO97/25295 discloses a device for aerobic treatment, conditioning and/or drying of a moist substantially solid or paste-like material that has a vessel with paired Venetian-blind-like walls, preferably with adjustable blades, and having air inlet and discharge chambers. WO2004/048868, which was published on 10 Jun. 2004, discloses a continuous gravity flow apparatus for heating and/or drying particulate material that is vertically disposed and has a top and a bottom with a flow passage for the particulate material between cylindrical outer and inner housings that have perforated wall sections.
There is a need for a method, apparatus and plant for drying particulate materials, such as pellets comprising low rank carbonaceous materials, which overcome or at least alleviate one or more disadvantages of the prior art. It is in view of this background that the present invention has been conceived, which may offer advantages over prior art approaches in terms of efficiency of drying, structural integrity of product produced and minimisation of capital, handling and operating costs, for example.